The present invention relates to methods and apparatuses for the nondestructive assessment of a metallic material. More particularly, the present invention relates to the nondestructive, in situ, material assessment of service-exposed combustion turbine blades.
Metallic components such as industrial gas turbine blades are used under elevated temperature creep conditions and are designed to withstand the maximum stress which can exist in the structure for the required design life. A safety factor is applied to this stress to allow for material variability and variations in the operating conditions. This design stress, if not exceeded, should preclude failure within the design life. In practice, however, the blade experiences considerable mechanical and thermal stresses which can cause damage such as creep (cavitation), microstructural embrittlement and cracking. This damage, if undetected, can result in an unexpected failure, costly repairs and severe damage to the turbine unit.
Periodically, during plant outages, combustion turbine blades that contain no obvious cracks are destructively evaluated in order to monitor and assess the response of blading material to service conditions. During these investigations, stress rupture tests and visual microstructure examinations are performed on selected blades to assess the state of the material and to estimate conditions to which the turbine blades are exposed during service operation. These destructive examinations consist of comparing pre-service and post-service material properties and microstructures. Material specifications serve as a guide in assessing remaining life and the comparison is used to determine whether remaining blades can be put back into service. The advantages of an accurate material assessment method include the avoidance of costly unscheduled outages and the ability to extend the useful life of the blade.
Although analyses of properties from post-service turbine blades provide useful information on life extension, the evaluation process is both time consuming and costly. In addition, the frequent removal of blades for destructive analysis is costly. In the case of combustion turbines, the operation cycle, type of fuel and environmental conditions can vary significantly from machine to machine. For instance, for a given turbine model, some blades may experience higher temperatures than others. As a result, blades can have different microstructures even though they have accumulated the same service exposure time.
Some combustion components operate at very high temperatures (metal temperature between 1500.degree. F. to 1800.degree. F. (815.degree. C. to 982.degree. C.)) and are subjected to different loading ad environmental conditions. FIG. 1 depicts a cross section of a typical combustion turbine blade airfoil 10. Such blades are produced from a high temperature creep resistant non magnetic nickel base superalloy.
The blade comprises a root (not shown) and the airfoil 10. The blade root is used to attach the blade to the turbine disc, and operates at relatively low temperatures, while the airfoil experiences much higher temperatures as it passes through the hot gases created by the combustion process. To keep the airfoil cool during operation, small radial cooling holes 10A, 10B, 10C, etc., are drilled along the length of the airfoil during the manufacturing process. During operation, air passes through these holes, cooling the blade from the interior to the exterior. As a result, large thermal gradients and stresses are created Which can significantly degrade the blade material with time.
When service exposed blades are removed and inspected, significant microstructural and material property differences are observed. These differences can be directly related to the blade's metal temperature and exposure time. In general, service blades which have experienced lower operating temperatures for shorter times have better material properties than those exposed to higher temperatures for longer times.
FIG. 2 depicts the cross-section of a blade's airfoil 10, including a temperature profile from a blade that was in service for 10,000 hours. As shown, the highest metal temperature occurs at the leading and trailing edges and at the concave surface (pressure side) of the blade. Near the centermost cooling holes 10D, 10E, 10F, 10G, lower metal temperatures occur, with the coldest area located in the center of the thick part of the airfoil. The microstructure in the latter area is similar to those of a pre-service blade, i.e., unaffected by service temperature.
Westinghouse Electric Corporation, the assignee of the instant application, has demonstrated that special nondestructive sensors can detect and characterize time-temperature dependent degradation such as creep (U.S. Pat. No. 4,746,858) and temper embrittlement (U.S. Pat. No. 4,528,856) in some materials. These Patents may be referred to for further background on the present invention.